1. Field of the Invention
The present invention generally relates to engineered microparticles, and more particularly, relates to an engineered, low-density microparticle with high chemical durability. The present invention also relates to methods and formulations for forming the microparticle and uses thereof.
2. Description of the Related Art
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Cenospheres are generally spherical inorganic hollow microparticles that are commonly found in fly ash produced as a by-product in coal-fired power stations. Cenospheres typically make up around 1-2% of the fly ash and can be recovered or “harvested” from the fly ash. These cenospheres derived from coal combustion are commercially available. The composition, form, size, shape and density of the coal-derived cenospheres provide particular benefits in the formulation and manufacture of many low-density products.
One of the characterizing features of the coal-derived cenospheres is their high chemical durability. This high chemical durability is understood to be due to the relatively low content of alkali metal oxides, particularly sodium oxide, in their composition. Accordingly, low-density composites produced from coal-derived cenospheres have the desirable properties of high strength to weight ratio and chemical inertness. Chemical inertness is especially important in Portland cement applications, where relative chemical inertness plays an important role in achieving highly durable cementitious products. Thus, harvested cenospheres from coal combustion have proven to be especially useful in building products and in general applications where they may come into contact with corrosive environments.
Despite the known utility of coal-derived cenospheres, their widespread use has been limited to a large extent by their cost and availability. The recovery of cenospheres in large quantities from fly ash is a labor intensive and expensive process. Although it is possible to increase the recovery of cenospheres from fly ash by modifying the collection process, the cost of improved recovery does not make this economically viable.
It may also be possible to alter combustion conditions in power stations to increase the yield of cenospheres in fly ash. However, combustion conditions in power stations are optimized for coal-burning rather than cenosphere production, and it is not economically viable to increase the yield of cenosphere production at the expense of coal-burning efficiency. Moreover, while the coal-derived cenospheres appear to be chemically durable in a cementitious environment, they still exhibit some degree of leaching in a caustic environment.
In addition to coal-derived cenospheres, the prior art also discloses incorporating synthetic glass microspheres in certain low-density composite materials. However, there are also disadvantages associated with the properties and/or methods of making these conventional synthetic glass microspheres. For example, an early method for manufacturing hollow glass microspheres involved combining sodium silicate and borax with a suitable foaming agent, drying and crushing the mixture, adjusting the size of the crushed particles and subsequently firing the particles. However, this method suffers from the use of borax, which is an expensive starting materials. Hence, the resulting microspheres are necessarily expensive. In addition, the product has poor chemical durability due to a high percentage of sodium oxide in the resulting glass composition.
Generally speaking, prior art methods for forming engineered expanded microparticles such as glass microspheres involve firing an inorganic material in the presence of a blowing, gasifying or foaming agent. Such blowing, gasifying or foaming agents are typically activated when the material from which the microparticle is produced is in an appropriate form, such as liquid. However, it is sometimes extremely difficult to match the blowing agent with the material from which the microparticle will be formed and using the blowing agent in the most efficient manner.
In addition, prior art methods for forming engineered expanded microparticles generally describe heating the starting materials to form a homogeneous melt prior to expanding the materials. However, a significant amount of the foaming agent could escape during the process, thus increasing the density of the foamed product.
In view of the foregoing, it will be appreciated that there is a need for a synthetic, low-cost microparticle engineered to have favorable physical and chemical properties for incorporation into low density composite materials. It is also desirable to have a system which allows a greater degree of control over the process of forming the engineered microparticles.